Converting between rotary and linear motion, and a sawing device

ABSTRACT

Apparatus for converting between rotary motion and linear motion includes a rotatable piece being rotatable about an axis; at least one member having a linking surface facing the rotatable piece and arranged for movement relative to the axis of the piece; and a means for linking the linking surface of the respective member and the surface of the rotatable piece, so that rotational motion of the rotatable piece causes linear movement of the at least one member, and/or linear movement of the at least one member causes rotational motion of the rotatable piece. A sawing device may include said apparatus for cutting by repetitive reciprocating cutting action. A saw blade may be configured for such a sawing device.

FIELD OF THE INVENTION

The invention relates to apparatus for converting rotary motion tolinear motion and/or linear motion to rotary motion. The invention alsorelates to a sawing device for cutting by repetitive reciprocatingcutting action of at least one saw blade on a surface of an object to becut. The invention also relates to a method of replacing a blade in asawing device, apparatus for a saw blade, and to a saw blade.

BACKGROUND

Plasterboard, also known as drywall, gypsum board or wallboard, is apanel made of gypsum plaster pressed between two thick sheets of paper.Plasterboard panels are typically mounted on structural members to makeinterior walls and ceilings of buildings. It is sometimes necessary,particularly when a building is erected, to cut openings in plasterboardto provide access to electrical switch and outlet boxes mounted on thestructural members. An opening may be cut before the plasterboard ismounted on the structural members, or it may be necessary to cut theopening while the plasterboard is in situ mounted on the structuralmembers.

The usual process of cutting openings comprises firstly marking on theplasterboard where the opening is to be cut. Then, a workman cuts theopening using a saw. The opening typically has rough edges. This processis time consuming, typically taking a practised workman 15 to 30minutes. The process is also error prone and typically makes a mess.Also, the saw can extend beyond the plasterboard to make contact with anobject on the far side of the plasterboard from the workman. This cancause damage to the object. Since there may be electrical wiring, thisalso presents danger to the workman.

A known document, U.S. Pat. No. 3,503,294, describes a sawing apparatus.This sawing apparatus includes a crank member that is coupled to arotational power source. Sawing action of four blades is driven byeccentric movement of the crank member. The sawing apparatus is unstablein use, which may lead to a user applying different blades to an objectto be cut at different pressures. This may disadvantageously lead to theobject being cut more quickly in some regions than in others. Inaddition, the eccentric motion of the crank causes vibration of thedevice. The issues of instability and vibration make the apparatusdifficult and unpleasant to use, and may lead to unsatisfactory andinaccurate cutting.

It is an object of the present invention to address these issues.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is providedapparatus for converting rotary motion to linear motion and/or linearmotion to rotary motion, comprising: a rotatable piece having a surface,the piece being rotatable about an axis thereof; at least one memberhaving a linking surface facing the surface of the rotatable piece andarranged for restricted movement at least in part laterally relative tothe axis of the piece; and located between the surface of the rotatablepiece and the or each reciprocating member, means for linking thelinking surface of the respective member and the surface of therotatable piece, wherein the means for linking, the surface of therotatable piece and the linking surface are configured to cooperate sothat rotational motion of the rotatable piece causes linear movement ofthe at least one member, and/or linear movement of the at least onemember causes rotational motion of the rotatable piece.

As will be clear from the following disclosure, although the inventorset out to devise an improved sawing device, he devised a new way toconvert between linear and rotary motion. The number of parts requiredis small, the apparatus can be easily and inexpensively manufactured,and the transmission efficiency is high.

There is also provided a sawing device for cutting by repetitivereciprocating cutting action of at least one blade on a surface of anobject to be cut, the sawing device comprising: the apparatus describedabove; a rotary drive shaft engaged with the rotatable piece to causerotation thereof; and at least one blade coupled to the or each memberlengthwise with the direction of linear movement of the member; whereinthe apparatus is arranged to convert rotational motion of the rotarydrive shaft to repetitive reciprocal movements of the or each member,which causes corresponding movement of the at least one blade.

The drawbacks to the sawing apparatus described in U.S. Pat. No.3,503,294 are thus addressed. Even pressure can easily be applied byeach blade to an object to be cut. The sawing device is stable to use,in particular since the conversion of rotary motion to reciprocatingmotion is achieved in a smooth, continuous process.

According to a second aspect of the present invention, there is provideda sawing device for cutting by repetitive reciprocating cutting actionof at least one saw blade on a surface of an object to be cut, thesawing device including a rotary drive shaft and means for convertingrotational motion of the rotary drive shaft to repetitive reciprocalmovements for imparting to the at least one blade, wherein the axis ofthe rotary drive shaft extends in or in the direction of the vicinity ofthe centre of mass of the sawing device.

According to a third aspect of the present invention, apparatus for asawing device, comprising: a support means having a slot therethrough;and a blade carrying means arranged on said support means, wherein thesupport means permits reciprocating movement of the blade carryingmeans, wherein the blade carrying means is arranged to attach to andcarry a blade and transmit a corresponding reciprocating action to theblade through the slot.

According to a fourth aspect of the present invention, there is provideda saw blade for attaching to a sawing device mentioned above, the bladecomprising a body having a cutting edge and a first attaching means forengaging with a corresponding second attaching means of a blade carryingmeans of the sawing device.

Such a saw blade enables a user of the sawing device to quickly andconveniently replace a blade, while also resulting in the blade beingheld very securely. Such an arrangement is particularly convenient oversaw blade attachment arrangements in which a blade must be attachedmanually at both its ends to blade holders.

BRIEF DESCRIPTION OF THE FIGURES

For better understanding of the present invention, embodiments will nowbe described, by way of example only, in which:

FIG. 1 is an exploded underside perspective view of a sawing deviceaccording to a first embodiment of the invention;

FIG. 2 is a perspective view of the saw device according to the firstembodiment in a partially assembled form, with an outer housing absent;

FIG. 3 is a perspective view of the sawing device according to the firstembodiment, without inner or outer housing; and

FIGS. 4A and 4B are perspective views of parts of the sawing device,including a rotatable cylindrical block;

FIG. 5 is a perspective view of a mounting plate for the sawing device,together with a mount for the cylindrical block shown in FIG. 4;

FIG. 6 is a perspective front view of a part of a blade carrying part ofthe sawing device for use in attaching the saw blade shown in FIG. 7 tothe rest of the sawing device;

FIG. 7 is a perspective front view of a saw blade for use in the sawingdevice;

FIG. 8 is an exploded view of parts of a sawing device in accordancewith a variant embodiment, including a blade carrying part and a blade;

FIG. 9 is a view of a mounting plate for use with the parts shown inFIG. 8;

FIG. 10 is an exploded view of the sawing device of the variantembodiment; and

FIG. 11 is a view of a disc for use in another embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Like parts are denoted by like reference numerals throughout.

By way of general overview of some embodiments, the sawing devicedescribed includes a rotary to linear motion converting arrangement. Theapplication of the converting arrangement is not limited to use in thesawing device; there are many applications for such convertingarrangements. Also, the converting arrangement may be used to convertlinear motion to rotary motion, as well as rotary motion to linearmotion.

Generally, the sawing device is suitable for cutting a predeterminedshape by simultaneous repetitive reciprocating cutting action of fourblades. The four saw blades are relatively disposed for simultaneousengagement of a surface of an object such as plasterboard to be cut.Preferably the surface is a substantially flat surface. The sawingdevice includes a rotary drive shaft and the motion convertingarrangement for converting rotational motion of the rotary drive shaftto repetitive reciprocal movements of the blades. In some embodiments,the central axis of the rotary drive shaft extends through or in thevicinity of the centre of mass of the sawing device.

Referring to FIG. 1, the sawing device comprises blades 10 a-f, a rotarydrive shaft 12 a, 12 b, an inner housing 14, an outer housing 16, ablade carrying parts 18 a-d carrying each blade 10 a-f, and a motionconverting arrangement, which will be described in greater detail belowand is largely hidden in FIG. 1, but includes a support means in theform of a mounting plate 20.

In FIG. 1 (and also in FIG. 3), the sawing device is shown with sixblades 10 a-f to illustrate how the blades can be fitted. In use, fourblades would be mounted in a generally square or rectangular formationto enable cutting of a corresponding shape.

The inner housing 14 and the outer housing 16 each has a centrallylocated aperture through which the rotary drive shaft 12 a, 12 b extendswhen the sawing device is assembled. The outer housing 16 includes aflange 16 a extending outwardly in a plane parallel to a plane of theedges of the blades 10 a-f. The flange 16 a is for pressing against asurface to be cut to limit depth of cutting. The flange 16 a also servesto steady the device when cutting.

The outer housing 16 has a spirit level 22 mounted on it runningparallel to the flange 16 a. The spirit level 22 facilitates location ofthe sawing device so that a shape in a desired orientation can be cut inan at least partially vertical surface. The desired orientation wouldnormally be such that edges of the shape are parallel or orthogonal tothe ground.

The inner housing 14 has four grooves 22 a-d therein in a generallysquare formation. When assembled, the inner housing 14 fits over themounting plate 20 with the rotary shaft 12 a, 12 b extending through theaperture in the inner housing 14, as shown in FIG. 2, so as to enclosethe motion converting arrangement (not shown in FIG. 2) and bladecarrying parts 18 a-d. The inner housing 14 attaches to the mountingplate 20 to substantially prevent relative movement. Various ways ofattaching the inner housing 14 and the mounting plate 20 will beapparent to persons skilled in the art, for example by use of screws andsuitably threaded apertures. When assembled, the outer housing 16 fitsover the inner housing 14.

A means for biasing (not shown) the inner housing 14 into the outerhousing 16 is located between the inner and outer housings 14, 16. Thus,since the mounting plate 20 is fixedly disposed relative to the innerhousing 14, the biasing means causes the blades 10 a-f to retract intothe outer housing 16 when not in use. A stopper means (not shown) isalso provided to limit the maximum extent that the mounting plate 20 andthus the blades can extend from the outer housing 16. The stopper means,in conjunction with the flange 16 a, limits the depth to which an objectcan be cut.

Each blade 10 a-f is a saw blade having a serrated cutting edge forcutting an object to which the cutting edge is applied when the blade 10a-f is engaged in a longitudinally directed, repetitive reciprocatingmotion. The blades may be formed of metal. Each blade 10 a-d includes afirst attachment means for engaging the blade to an associated bladecarrying part 18 a-d, which has a corresponding second attachment forengaging with the first attachment means. Exemplary first and secondattachment means are described in detail below.

Referring also to FIG. 3, the motion converting arrangement includes acylindrical block 24 mounted on the mounting plate 20, a flanged bushing23, the mounting plate 20, four reciprocating walls 26 a-d each mountedon the mounting plate 20, and, for each reciprocating wall 26-d, alinking means in the form of a ball bearing for transferring force fromthe cylindrical block 24 to the reciprocating walls 26 a-d.

Each reciprocating wall 26 a-d is joined to or formed with or attachedor otherwise coupled to a respective one of the blade carrying parts 18a-d so that a movement of each reciprocating wall 26-d is transmitted tothe corresponding blade carrying part 18 a-d.

The rotary drive shaft 12 a, 12 b has two different diameters. A firstend 12 a of the rotary drive shaft 12 a, 12 b is for engagement with arotary power source (not shown), for example a drill motor, and has adiameter and cross-section suitable for engagement by the rotary powersource. A second end 12 b of the rotary drive shaft 12 of largerdiameter than the first end 12 a extends into the cylindrical block 24and engages with the cylindrical block 24 in a conventional manner, suchthat rotation of the rotary drive shaft 12 drives rotation of thecylindrical block 24 about the axis of the cylindrical block 24.

As also shown in FIGS. 4A and 4B, the cylindrical block 24 has acylindrical surface 24 a having a continuous non-linear groove orchannel 28 extending in a regular zigzag manner circumferentially aroundthe cylindrical surface 20 a. The groove has four peaks and fourtroughs. The groove 28 is shaped to carry a ball bearing 30 a, 30 d foreach reciprocating wall 26 a-d (ball bearings are indicated at 30 a and30 d in FIG. 4B and are absent in FIG. 4A; ball bearings 30 b and 30 care hidden in FIG. 4B) such that the ball bearings 30 a-d do not slipfrom the groove 28. In alternative embodiments, the groove 28 may have adifferent number of peaks and troughs, which changes the ratio ofrotations to repetitive movements of the walls 26 a-d.

The cylindrical block 24 has a cylindrical recess 25 in its baseextending coaxially with the axis of the cylindrical block 24, to enablemounting of the cylindrical block 24.

The flanged bushing 19 is tight fitted against the inner housing 14,when the sawing device is assembled. Bearings are preferably locatedbetween the flanged bushing 19 and the cylindrical block to reducefriction.

Referring to FIG. 5, the mounting plate 20 is rectangular having twolong edges 32 a, 32 b and two shorter edges 32 a, 32 b. The mountingplate 20 has a square groove 34 a-d therein. The groove 34 a-d extendsto approximately a depth of half of the thickness of the mounting plate20. Two groove portions 34 a, 34 b of the square groove 34 a-d areparallel to the two long edges 32 a, 32 b and the other two grooveportions 34 c, 34 d of the square groove 34 a-d are parallel to the twoshorter edges 32 c, 32 d. The distance between each of the two grooveportions 34 a, 34 b of the square and a respective closest one of thetwo long edges 32 a, 32 b is the same. The distance between each of thetwo other groove portions 34 c, 34 d of the square aperture and arespective closest one of the two shorter edges 32 c, 32 d is also thesame.

The mounting plate 20 has six slots 36 a-f located therethrough, viawhich each blade carrying member 18 a-d and associated blades 10 a-fengage. Four of the six are respectively located adjacent and parallelto a respective groove portions 34 a-d of the square groove 34 a-d. Twoof the six are respectively located adjacent and parallel to the twoshorter edges 32 c and 32 d of the mounting plate 20. The slots 36 a-fare located so that blades 10 a-f are carried in positions so that asquare or rectangular cut-out can be produced by simultaneous cuttingaction of the blades. The blades 10 a, 10 b extend and cutlongitudinally with the slots.

The mounting plate 20 has a mount for the cylindrical block 24 in theform of a cylindrical pin 39. When the sawing device is assembled, thecylindrical pin 39 extends into the recess in the cylindrical block 24such that the cylindrical block 24 and the pin 39 are coaxial. Anannular bearing piece may be located between the surface of the recessand the pin 39 to reduce friction. The cylindrical block 24 is thusmounted to spin on the cylindrical pin.

Returning to FIG. 3, reciprocating wall 26 a-d has an L-shaped slidingportion 42 a-d, which sits in the square groove 34 a-d and is able toslide therein. Each reciprocating wall 26 a-d is thus configured toengage with a respective one of the grooves 22 a-d in the inner housing14 so as to allow sliding movement of the reciprocating wall, but toprevent movement in other directions. The sliding movement isapproximately tangential to the cylindrical block 20. To facilitatesliding movement in the grooves 22 a-d, an upper surface of eachreciprocating wall 26 a-d has an elongate recess therein in which aplurality of ball bearing 55 a-d are located. When the inner housing 14is located over the mounting plate 20 so as to enclose the reciprocatingwalls 26 a-d, the ball bearings protrude into the grooves 22 a-d in theinner housing 14, such that the reciprocating walls 26 a-d can slidebetween the grooves 22 a-d and groove portions 34 a-d in a low frictionmanner.

Each reciprocating wall 26 a-d has a linking surface 44 a-d facing thecylindrical block 24. Each linking surface 44 a-d has a respective forcetransmission groove 46 a-d therein extending diagonally in the linkingsurface, that is, neither in a direction completely parallel to thedirection of sliding movement with respect to the sliding surface nor ina direction completely perpendicular to the direction of slidingmovement.

The surface 24 a of the cylindrical block 24 and each wall 26 a-d arespaced such that the respective ball bearing 30 a, 30 b engages in therespective force transmission groove 46 a-d. The channel 28 and eachforce transmission groove 46 a-d are arranged, together with therespective ball bearing 30 a, 30 b, to cooperate so that rotation of thecylindrical block 20 causes the ball bearing to run back and forth inthe force transmission groove, which requires the respectivereciprocating wall 26 a-d to slide back and forth along the respectivegroove portion 34 a-d.

The blade carrying parts 18 a-d comprise two first blade carrying parts18 a, 18 b and two second blade carrying parts 18 c, 18 d. The firstblade carrying parts 18 a, 18 b are each configured to attach to arespective blade 10 a, 10 b through a respective associated one of theslots 36 a, 36 b between the square groove 34 a-d and a one of thelonger edges of the mounting plate 20. The first blade carrying parts 18a, 18 b are each attached to a respective adjacent reciprocating wall 26a, 26 b, such that reciprocating movement of the walls 26 a, 26 b causescorresponding reciprocating movement of the respective blade carryingpart 18 a, 18 b and thus the associated blade 10 a, 10 b.

The first blade carrying parts 18 a, 18 b are each in the form ofattachment means comprising a plate attached to a surface of theassociated reciprocating wall 26 a, 26 b at a first end thereof withscrews, for example. The plate extends through the associated slot 36a-d, engaging with the blade 10 a, 10 b that it carries. The plate andthe manner of engagement with the blade that each plate carries will bedescribed in greater detail below.

The second blade carrying parts 18 c, 18 d are each capable of carryingtwo blades, although only one blade is carried at any one time. Each ofthe second blade carrying parts 18 c, 18 d attaches to a respectiveblade 10 c-f through a slot 36 c-f in the mounting plate 20. The secondblade carrying parts 18 c, 18 d include a sliding plate 17 a, 17 bextending from the associated reciprocating wall 26 c, 26 d over themounting plate 20 to the slots 36 c, 36 d adjacent the short edges ofthe mounting plate 20. The sliding plate 17 a, 17 b is joined to theassociated reciprocating wall 26 c, 26 d, such that movement of the wall26 c, 26 d causes corresponding sliding movement of the sliding plate onthe mounting plate 20.

Each sliding plate 17 a, 17 b has a pair of slits 43 a-d thereinaligning with a respective one of the slots 36 c-f so that an attachmentmeans of the second blade carrying part 16 c, 16 d can attach to theblade 10 c-f that it carries. The attachment means in the same form asthe attachment means of the first blade carrying parts 18 a, 18 b. Theattachment means that extends through the slit 43 d and slot 36 dadjacent the groove portion 34 d is attached at one end thereof to arespective reciprocating wall 26 d and extends through the slit and slotfor engagement with a blade. The attachment means of the second bladecarrying means 26 c, which extends through the slit 43 c and slot 36 c,is attached and extends in an analogous way.

Each second blade carrying means 16 c, 16 d also includes a wall (notshown) extending perpendicularly from the sliding plate 17 a, 17 badjacent the shorter edges 32 c, 32 d of the mounting plate 20.Attachment means (not shown) in the same form as the attachment means ofthe first blade carrying parts 16 a, 16 b is attached at a first end tothe wall and extends through a one of the slits 43 e, 43 f andcorresponding slots 36 e, 36 f for engaging and carrying a blade.

In the present embodiment, each sliding plate 17 a, 17 b, the respectivereciprocating walls 26 a-d and the sliding portion 42 a-d are formed ofa single piece. In alternative embodiments, the reciprocating wall 26 c,26 d can be separate from the corresponding blade carrying part 16 a-d,but coupled to transfer repetitive reciprocating motion thereto, so asto impart such motion to the four of the blades 10 a-f that areattached.

Referring to FIG. 6, each attachment means, indicated at 49, is forengaging with a blade and comprises a plate, as already mentioned. Theplate is attached to one of the reciprocating walls 26 a-d, or the otherwall extending perpendicularly from the sliding plate, at a first endthereof, and forms an arm for extending through a one of the slots 36a-f, and where appropriate one of the slits 43 c-f. The arm ends in abulbous portion 49 a in the shape of a dovetail or the like.

Referring to FIG. 7, each blade 10 a-d has blade attachment means in theform of a recessed region 52 and a pair of parallel limbs 51 a, 51 bleading from the recessed region 52. The shape of the region between thelimbs 51 a, 51 b together with the recessed region 52 is the same shapeas that of the arm 49. The limbs 51 a, 51 b can be inserted in theassociated slot over the arm 49 and the arm 49 flexed to allow this,until the end 49 a of the arm snap-fits into the recessed region. Inthis position the attachment means 49 and the blade are attached. Theparallel limbs 51 a, 51 b and the arm are respectively shaped sorespective adjacent edges fit flush. This prevents movement of theblades 10 a-d relative to the blade carrying part 16 a-d.

Each of the blades 10 a-10 d is arranged having respective first andsecond ends, the first end of each blade lying adjacent to the secondend of the blade displaced next to it. Reciprocating movement of thereciprocating wall 26 a-d causes the blade carried by the respectiveattached blade carrying part 16 c-d to move in a repetitivereciprocating motion, thereby to cause the blade to move in a repetitivereciprocating motion.

Each blade 10 a-d is held in a fixed position by the blade carryingpart, such that they simultaneously engage a surface of an object to besawed. The slots 36 a-f are sized to allow reciprocating movement of thecorresponding plate 40 a-d.

Typically it is desired to cut a rectangular opening of a particularsize, for example in plasterboard to allow electrical wiring for adouble plug socket to pass. Blades 10 a-d enable this. However, it isalso often desired to cut a square opening of a particular size, forexample in plasterboard to allow electrical wiring for a single plugsocket to pass. The longer blades 10 a, 10 b can be replaced with bladesof the same size as the blades 10 c, 10 d and the blades 10 c and 10 dremoved and replaced with blades 10 e and 10 f to enable this.

Each of the blade carrying parts 18 a-d, the reciprocating plates 40 a-dand the projections 42 a-d have dimensions permitting reciprocatingmotion without collision with an adjacent part.

In operation, the rotary drive shaft 12 a, 12 b rotates about itscentral axis. Since each ball bearing 30 a, 30 b is restricted inmovement by the respective force transmission groove 46 a-d, each ballbearing 30 a-30 d can move only in the groove. Thus rotation of therotary drive shaft 12 a, 12 b causes oscillatory movement of the ballbearing 30 a, 30 b parallel to the axis of the drive shaft 12 a, 12 b inthe groove 28. In order for such movement to occur, the ball bearing 30a, 30 b must move a same distance in the respective force transmissiongroove 46 a-d. This movement imparts repetitive reciprocating movementto the reciprocating wall 26 a-d, wherein the sliding portion 42 a-dmoves in a side of the square groove 34 a-d. The reciprocating walls 26a-d move laterally and approximately tangentially relative to the axisof the cylindrical block 24. Such movement of the reciprocating wall 26a-d causes corresponding movement of the blade carrying member 18 a-d,which in turn causes corresponding movement of the respective blade 10a-d. The movement of the reciprocating walls 26 a-d is such that thesecond end of each blade approaches the first end of each blade, and thefirst end of each blade approaches the second end of each blade in analternating sequence.

The sawing device is preferably configured such that the path of eachblade substantially meets the path of the respective adjacent blade toform a notional apex. Thus, a portion of an object can be cut out.Notably, if the length of the blades is such that the respective pathsdo not quite meet, the portion of an object can still be pushed out.Preferably the paths of the blades are such that they substantiallymeet, that is, they at least come sufficiently close to enable a cutshape to be pushed or tapped out of a cut object. Also, if the path of ablade extends to cross the path of an adjacent blade, this is acceptablesince a portion of an object will still be able to be cut out. Thus thepaths of the ends of each blade may overlap the path of the ends of nextadjacent blades, but not such that reciprocating movement of an adjacentblade is impeded.

A saw blade may be replaced by flexing the arm 49 to allow release ofthe blade 10 a-f. A new saw blade may be inserted by flexing the arm 49and sliding the limbs of the blade attachment means over the arm untilthe arm 49 and the limbs engage.

Several parts in the sawing device slide over one another at high speed.Suitable lubricant is provided between such parts, as will be apparentto the skilled person.

In a variant embodiment, the blades 10 a-f, the wall 26 a-d, the bladecarrying parts 18 a-d differ to those described above, as indicated inFIGS. 8, 9 and 10. The blade carrying part is partly formed in the backof the wall 26 a-d, in that the wall is recessed to receive the arm 49,and the limbs 51 a, 51 b of the blade 10. The arm 49, as mentionedabove, has a bulbous end 49 a, and the other end thereof forms ahammerhead 50. A first part of the recess in the wall 26 is shaped toreceive the hammerhead 50. The recess is also shaped to include a neckthrough which the rest of the arm 49 depends. The arm 49 extends throughthe neck into a larger part of the recess, which is also shaped toreceive the limbs 51 a, 51 b. The larger part of the recess has twodepths, arranged such that the arm 49 can be pushed deeper than thelimbs 51 a, 51 b. The limbs 51 a, 51 b can gain entry to the largerparty of the recess when the arm is depressed. When the arm 49 isreleased, the arm 49 engages between the limbs 51 a, 51 b, including thebulbous part 49 a of the arm engaging into the recess 52 from which thelimbs extend. As can be understood from FIG. 8, an outer surface of thearm and limbs 51 a, 51 b, and the wall 26 are coplanar when the blade issecured in the attachment means.

A cover plate 53 attaches to the back of the wall 26 by means of screwsthrough holes 56 in the cover plate 53, although other means ofattaching the cover plate are suitable. The blade 10 can be attached anddetached while the cover plate 53 is in place, but depressing the arm 49can be depressed by pressing through an aperture 54 in the cover plate53.

Referring also to FIG. 9, the mounting plate and wall 26 is modified toretain ball bearings 56 between them. In this case, the mounting plateincludes grooves 59 a-d in place of a previously mentioned square groovein which the ball bearings 56 move in a reciprocating linear motion. Thewall 26 does not include a sliding portion 42.

As can be seen in FIG. 10, in this variant embodiment the sawing deviceincludes a sleeve 61 located around the cylindrical block. The sleeve 61has four elongate slots therein. The slots are sized and located suchthat a ball bearing can move in each of the slots. The sleeve 61 therebyguides movement of the ball bearings 30 a-d.

Four bolts and bolt sleeves are also shown in FIG. 10, which attach theinner housing 14 to the mounting 20.

The variant embodiment operates in a similar manner to that alreadydescribed. To replace a saw blade, the arm 49 is flexed by putting apointed object through the aperture 54 of the cover plate 53.

In other embodiments, rotational motion of a shaft may be converted tolinear repetitive reciprocating movement, or vice versa, using the sameprinciple of two parts (wall and block) having facing surfaces, eachsurface having a groove therein and a ball bearing part located in eachgroove, wherein the grooves and the ball bearing are configured tocooperate to impart force from one groove to another. However in suchother embodiments, the block is not cylindrical. For example the blockmay be frusto-conical. The block may be any shape in which the radialdistance of the groove from a central axis is constant. The shape of thewall 26 a-d may be modified so that the groove therein is a constantdistance from the groove of the block. In embodiments, the grooves maynot be spaced by a constant distance and the ball bearing may be out ofthe groove in the wall for a part of its oscillation.

In another embodiment, instead of a cylindrical block, a disc includinga circular or annular surface is provided. The disc has a continuousgroove therein extending in an oscillatory manner in a loop on a surfacethereof. Referring to FIG. 11, the circular disc 60 can be attached to arotatable shaft (not shown) by engagement in an aperture 62 through thedisc 60. The rotatable shaft is attached so that, when the shaftrotates, the disc rotates about its centre in a planar manner, the shafthaving an axis normal to the plane of the disc. The disc 60 has a groove64 therein extending continuously around the axis between the aperture60 and an edge 64 of the disc 60. In an arrangement for convertingrotational motion to linear motion, the disc is mounted on the rotatabledrive shaft and the shaft is mounted on a support such that the disc canrotate. A wall, like the wall 26 described above is also mounted on thesupport, the support and the wall are configured so that movement of thewall is restricted to movement in a linear, reciprocating repetitivemanner in a direction lateral to the axis of the rotatable shaft. Asurface of the wall has a groove therein, located diagonal in relationto the direction of movement of the wall, and which faces the groove 64of the disc 60 and is close thereto. A ball bearing is part located inboth grooves to enable transmission of force.

In operation, rotation of the disc causes linear reciprocating movementof the wall in a manner similar to that described above. The ballbearing is caused to oscillate back and forth in a substantially radialdirection on the disc due to the restricted ability of the wall to move.This oscillating movement causes the wall to move at least partlylaterally relative to the axis and in a direction spaced from the planeof the disc. The arrangement including the disc may be included in asawing device, whereby rotational movement of a drive shaft is convertedto linear repetitive reciprocating movement of saw blades.

As well as enabling converting of rotary to linear motion, thearrangement may, additionally or alternatively, be used to convertlinear motion to rotary motion of the shaft.

It will be understood by the skilled person that various modificationsare possible to the embodiments described above.

The shape to be cut out of an object, for example a plasterboard, can bepredetermined to be other than a rectangle or a square. For example, thesawing device can be modified to carry three, five or six, seven oreight blades, or a greater number, with a corresponding number of bladecarrying parts, to enable a triangle, pentagon, hexagon, heptagon oroctagon to be cut out. Other shapes could be cut out with a previouslymentioned number of blades, for example the sawing device could beconfigured for cutting of a rhombus with four blades. With respect touse of the word rectangular in the claims, it should be understood thatthe term “square” is a special case of a “rectangle”.

The device may also have just two blades, preferably of the same lengthand spaced in a parallel manner by a distance equal to their length.This could be used to achieve cutting of a square or rectangle, andwould advantageously mean that the device could have fewer parts.

Also, the blade carrying parts might be mounted on the mounting platesuch that the motion converting arrangement causes reciprocating motionback and forth on a curved path. Thus, the sawing device can beconfigured such that, with curved blades, circular holes could be cut.

In a modification, the sawing device can be modified so that one linkingmeans, for example in the form of one ball bearing, causes reciprocatingmotion of more than one blade.

In alternative embodiments, the number of peaks and the number oftroughs can each be equal to or a multiple of the number of blades, sothat when a first end of one blade moves towards a second end of anadjacent blade, the second end of the adjacent blade moves away from thefirst end of the one blade.

The linking means described above linking the bearing surface 24 a ofthe cylindrical block 24 and the linking surface of the reciprocatingwall 26 a-d is a ball bearing which runs in grooves in the bearingsurface and the linking surface, but other ways of linking thecylindrical block and the blade carrying parts 18 a-d are possible. Forexample, a rail and runner arrangement may alternatively be used.

In modifications, the non-linear groove may not be of a zigzag form, butanother kind of oscillation. The precise kind of oscillation can beoptimised for best cutting action.

Various ways of adapting the blades and the blade carrying means so thatthe blades are fixedly held relative thereto are possible. For example,pairs of aligned slits and aligned slots may be present where singleslits 36 and slots 43 are provided in the embodiments described above.The blades may be adapted to attach to attachment means of the sawingdevice via the pairs of slits and slots in any kind of snap-fit manner.

The sawing device described above is described as a device for cuttingplasterboard. It will be appreciated by the skilled person that thesawing device could be applied to cut the surface of objects other thanplasterboard. For example, the sawing device can be used to cut variouskinds of sheet material, such as sheets or panels of wood, such asplywood or chipboards, plastic panels or conduits.

The sawing device may be provided with an elongate member attached orattachable to the outer housing 16. The member is of a predetermined oradjustable length, and can be positioned to enable the height from theground of a cut-out in a wall or largely vertical wall or panel to bepredetermined.

The applicant hereby discloses in isolation each individual featuredescribed herein and any combination of two or more such features, tothe extent that such features or combinations are capable of beingcarried out based on the present specification as a whole in the lightof the common general knowledge of a person skilled in the art,irrespective of whether such features or combinations of features solveany problems disclosed herein, and without limitation to the scope ofthe claims. The applicant indicates that aspects of the presentinvention may consist of any such individual feature or combination offeatures. In view of the foregoing description it will be evident to aperson skilled in the art that various modifications may be made withinthe scope of the invention.

The invention claimed is:
 1. An apparatus for converting rotary motionto reciprocating motion, comprising: a rotatable piece having a surface,the rotatable piece being rotatable about a central axis thereof and thesurface having a first cooperating portion in or on the surface; amember having a linking surface facing the surface of the rotatablepiece and arranged for reciprocating movement; the reciprocatingmovement of the member being along a path which is spaced laterallyapart from and which is non-parallel to the central axis of therotatable piece; and located between the surface of the rotatable pieceand the member, a link configured to link the linking surface of themember and the surface of the rotatable piece, wherein the linkingsurface of the member has a second cooperating portion extendingdiagonally on or in the linking surface relative to a direction of thereciprocating movement of the member, and wherein the link links thefirst cooperating portion to the second cooperating portion; wherein thelink, the first cooperating portion and the second cooperating portionare configured to cooperate so that rotational motion of the rotatablepiece in a direction of rotation causes the reciprocating movement ofthe member repetitively; and wherein the first cooperating portion is anon-linear continuous first groove, the second cooperating portion is asecond groove, and the link comprises a bearing located partially in thefirst and second grooves.
 2. The apparatus of claim 1, wherein thereciprocating movement of the member is repetitive reciprocating linearmovement.
 3. The apparatus of claim 1, wherein the rotatable piece ismounted on a support so as to allow rotation of the rotatable pieceabout the central axis, and wherein the member is also mounted on thesupport, the support being configured to restrict the reciprocatingmovement of the member to a linear path which is spaced from and atleast in part transverse relative to the central axis of the rotatablepiece.
 4. The apparatus of claim 1, wherein the linking surface and thesurface of the member are respectively spaced to retain said bearing insaid first and second grooves.
 5. The apparatus of claim 1, wherein saidfirst cooperating portion extends circumferentially around the centralaxis of the rotatable piece, wherein the radial distance of the firstcooperating portion from the central axis is constant.
 6. The apparatusof claim 1, wherein the rotatable piece is substantially cylindrical,and wherein said surface of the piece is the circumferential surface ofthe substantially cylindrical piece.
 7. The apparatus of claim 6,wherein the reciprocating movement is approximately tangential to thecircumferential surface or is in a plane parallel to a plane tangentialto the circumferential surface.
 8. An apparatus for converting rotarymotion to reciprocating motion, comprising: a rotatable piece having asurface, the rotatable piece being rotatable about a central axisthereof and the surface having a first cooperating portion in or on thesurface; a member having a linking surface facing the surface of therotatable piece and arranged for reciprocating movement thereciprocating movement of the member being along a path which is spacedlaterally apart from and which is non-parallel to the central axis ofthe rotatable piece; and located between the surface of the rotatablepiece and the member, a link configured to link the linking surface ofthe member and the surface of the rotatable piece, wherein the linkingsurface of the member has a second cooperating portion extendingdiagonally on or in the linking surface relative to a direction of thereciprocating movement of the member, and wherein the link links thefirst cooperating portion to the second cooperating portion; wherein thelink, the first cooperating portion and the second cooperating portionare configured to cooperate so that rotational motion of the rotatablepiece in a direction of rotation causes the reciprocating movement ofthe member repetitively; wherein the member comprises a plurality ofmembers; and the link comprises a plurality of links; and wherein eachof the plurality of members is configured to cooperate with acorresponding one of the plurality of links so that rotational motion ofthe rotatable piece in the direction of rotation causes thereciprocating movement of the plurality of members repetitively, or thereciprocating movement of the plurality of members repetitively causesrotational motion of the rotatable piece in the direction of rotation.9. An apparatus for converting rotary motion to reciprocating motion,comprising: a rotatable piece having a surface, the rotatable piecebeing rotatable about a central axis thereof and the surface having afirst cooperating portion in or on the surface; a plurality of memberseach having a linking surface facing the surface of the rotatable pieceand arranged for reciprocating movement, the reciprocating movementbeing along a path which is spaced laterally apart from the central axisof the piece and which is non-parallel to the central axis of the piece;and located between the surface of the rotatable piece and the pluralityof members, a link for each member, configured to link the linkingsurface of the respective member and the surface of the rotatable piece,wherein the linking surface of each member has a second cooperatingportion extending diagonally on or in the linking surface relative to adirection of the reciprocating movement of the respective member, andwherein the link links the first cooperating portion to the secondcooperating portion; wherein each link, the respective first cooperatingportion and the respective second cooperating portion are configured tocooperate so that rotational motion of the rotatable piece in adirection causes the reciprocating movement of the plurality of membersrepetitively.